Lithium titanate, electrode active material and electricity storage device each comprising the same

ABSTRACT

Disclosed is lithium titanate having excellent rate properties and useful for electricity storage devices, which is produced by preparing lithium titanate secondary particles that are aggregates of lithium titanate primary particles and forming at least macro-pores on the surfaces of the secondary particles. The lithium titanate can be produced by a process which comprises drying and granulating a slurry comprising crystalline titan oxide, a titanic acid compound and a lithium compound and firing the granulated product to thereby produce lithium titanate secondary particles, wherein (1) the crystalline titan oxide to be used comprises at least two types of crystalline titan oxide particles having different average particle diameters from each other, and/or (2) the crystalline titan oxide is used in an amount at least four-fold larger than that of the titanic acid compound in terms of TiO 2  content by weight. The lithium titanate can achieve a satisfactory level of charge-discharge capacity for practical uses when used in a electricity storage device without requiring the use of a carbon-containing substance, such as carbon black, acethylene black or Ketjen black, as an electrically conductive material in combination, in spite of a fact that lithium titanate has an electrically insulating properties by nature.

This is a divisional application of U.S. application Ser. No. 13/321,973filed Nov. 22, 2011, which is a National Phase application ofPCT/JP2010/058815 filed May 25, 2010.

TECHNICAL FIELD

The present invention relates to a lithium titanate having goodproperties for battery particularly excellent rate property and aprocess for production of the same. The present invention also relatesto an electrode active material comprising the lithium titanate, and anelectricity storage device using an electrode comprising the electrodeactive material.

BACKGROUND ART

Lithium secondary batteries have high energy density and good cycleperformance. Accordingly, recently, these have been rapidly used assmall-sized batteries for a power supply for portable devices or thelike. On the other hand, development for large-sized batteries for theelectric power industry, automobiles and the like has been demanded.These large-sized lithium secondary batteries need to comprise anelectrode active material having long-term reliability and high inputand output properties. Particularly, for the negative electrode activematerial, lithium titanate having high security, long life, andexcellent rate property has been expected, and a variety of lithiumtitanate has been proposed for the electrode active material. Forexample, a lithium titanate is known which is granulated into sphericalsecondary particles to improve packing properties, and thereby toimprove battery properties (Patent Documents 1 and 2). Such lithiumtitanate secondary particles are produced by granulating with drying atitanium compound and a lithium compound, and firing the granulatedproduct. Further, in order to improve the discharge capacity of thelithium titanate secondary particles, a process in which a slurrycomprising crystalline titanium oxide, a titanic acid compound, and alithium compound is granulated with drying, and then the granulatedproduct is heated and fired employs a process such as a process ofpreparing the slurry by adding a crystalline titanium oxide and atitanic acid compound to a solution of a lithium compound preheated to50° C. or more (Patent Document 3), or a process of preparing the slurryat a temperature of less than 45° C. (Patent Document 4). On the otherhand, a technique is known in which lithium titanate is crushed andfired again to form pores having an average pore size in the range of 50to 500 Å on the surface of the particle of lithium titanate, therebyimproving high current properties and cycle performance (Patent Document5).

CITATION LIST Patent Documents

PATENT DOCUMENT 1: JP 2001-192208 A

PATENT DOCUMENT 2: JP 2002-211925 A

PATENT DOCUMENT 3: JP 2005-239460 A

PATENT DOCUMENT 4: JP 2005-239461 A

PATENT DOCUMENT 5: JP 2007-18883 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention provides a lithium titanate having good propertiesfor battery particularly excellent rate property and a process forproduction of the same.

Means for Solving the Problems

As a result of extensive research, the present inventors found out thatthe secondary particle of a lithium titanate having at least macroporeson the surface thereof has more excellent rate property, and that such alithium titanate can be obtained as follows: two or more kind ofparticles are used as a crystalline titanium oxide, or the crystallinetitanium oxide and a titanic acid compound are blended in a specificratio at the previous mentioned process of drying and granulating aslurry comprising the crystalline titanium oxide, a titanic acidcompound, and a lithium compound, and firing the granulated product toobtain the secondary particles of the lithium titanate. Thus, thepresent invention has been completed.

Namely, the present invention is a lithium titanate comprising asecondary particle of aggregated lithium titanate primary particles andhaving at least macropores on the surface of the secondary particle, anda process for production of a lithium titanate, comprising the steps of:drying and granulating a slurry comprising a crystalline titanium oxide,a titanic acid compound, and a lithium compound; and firing a resultantproduct to obtain lithium titanate secondary particles, wherein (1) thecrystalline titanium oxide comprising at least two kind of crystallinetitanium oxide particles having different average particle sizes isused, and/or (2) the amount of the crystalline titanium oxide to be usedis more than 4 times larger than that of the titanic acid compound inthe weight ratio in terms of TiO₂.

Advantage of the Invention

An electricity storage device using the lithium titanate according tothe present invention for an electrode active material has good batteryperformances, particularly excellent rate property.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows adsorption and desorption isotherms in Example 1 (SampleA).

MODE FOR CARRYING OUT THE INVENTION

The present invention is a lithium titanate comprising a secondaryparticle of aggregated lithium titanate primary particles and having atleast macropores on the surface of the secondary particle. In thepresent invention, the lithium titanate is the secondary particle.Accordingly, depressions and projections, and gaps between the primaryparticles are formed on the surface of the secondary particle. Thereby,the lithium titanate can have a larger contact area with an electrolytesolution to increase the amount of lithium ions adsorbed and desorbed.In addition, the depressions and projections, the gaps between theprimary particles, and the like on the surface of the secondary particleform macropores, i.e., pores having pore size of 50 nm or more.Accordingly, the lithium titanate according to the present invention hasless load in adsorption and desorption of lithium ions than those havingmacropores with a pore size of 2 nm or less or mesopores with a poresize of 2 to 50 nm. For this reason, it is presumed that excellent rateproperty can be obtained. Usually, the pore size of a powder isdetermined as follows: the nitrogen adsorption and desorption isothermsdetermined by the nitrogen adsorption method are analyzed by an HKmethod, a BJH method or the like to determine pore distribution, andusing the total volume of pores calculated from the pore distributionand the measured value of the specific surface area, the pore size ofthe powder is determined. It is said, however, that these methods canmeasure the pore sizes of the micropore and the mesopore relativelycorrectly while the measured value of the pore size of the macropore hasvery low accuracy. On the other hand, it is said that in the adsorptionand desorption isotherms, a larger amount of nitrogen adsorbed at ahigher pressure in the relative pressure indicates presence of themacropore. In the present invention, in the case where the amount ofnitrogen adsorbed at a relative pressure of 0.99 (written asV_(a(0.99))) is 50 cm³ (STP)/g or more, it is determined that thelithium titanate has macropores. In the present invention, “cm³ (STP)/g”represents a value obtained by converting the amount of nitrogenadsorbed and desorbed into a volume in a standard state (temperature of0° C., atmospheric pressure of 101.3 KPa). More preferably, V_(a(0.99))is at least 55 cm³ (STP)/g.

Further, if few micropores and mesopores exist on the surface of thesecondary particle, load is further reduced in adsorption and desorptionof lithium ions, leading to more excellent rate property. It is saidthat the existence of few mesopores and micropores exist is shown by, inthe adsorption and desorption isotherms, a small amount of nitrogenadsorbed at a low pressure in the relative pressure, and no remarkabledifference in shape between the adsorption isotherm and the desorptionisotherm, namely, no occurrence of hysteresis between the adsorptionisotherm and the desorption isotherm. No occurrence of hysteresis can bespecifically shown by that the difference (written as ΔV_(d-a)) betweenthe amount of nitrogen desorbed and the amount of nitrogen adsorbed isvery small when measurement is made at an interval of the relativepressure of 0.05 in the range of 0.45 to 0.90, for example. In thepresent invention, it is determined that the surface of the secondaryparticle has neither micropore nor mesopore in the case where the amountof nitrogen adsorbed at a relative pressure of 0.50 (written asV_(a(0.50))) is 10 cm³ (STP)/g or less, and ΔV_(d-a) does notcontinuously take values of 5 cm³ (STP)/g or more, namely, the values ofΔV_(d-a) at the continuing two or more measurement points are not 5 cm³(STP)/g or more. V_(a(0.50)) is more preferably 8 cm³ (STP)/g or less.More preferably, ΔV_(d-a) does not continuously take values of 3 cm³(STP)/g or more.

Preferably, the average particle size of the secondary particle (50%median particle size according to the laser scattering method) is in therange of 0.5 to 100 μm from the viewpoint of packing properties. Fromthe viewpoint of battery properties, the shape of the secondary particleis preferably isotropic, and more preferably spherical or polyhedral.The primary particle that forms the secondary particle is notparticularly limited. The primary particle preferably has an averageparticle size (50% median particle size according to the electronmicroscopy) in the range of 0.01 to 2.0 μm because the particle size ofthe secondary particle in the range is easily obtained. The primaryparticle preferably has an isotropic shape such as a spherical orpolyhedral shape because the secondary particle of an isotropic shape iseasily obtained. The secondary particle is in a state where the primaryparticles are strongly bound to each other. The primary particles arenot agglomerated by interaction between particles such as a van derWaals force, nor mechanically press compacted. Accordingly, thesecondary particles are not easily broken in ordinary mechanicalcrushing that is industrially used, and most thereof remains as thesecondary particle.

The lithium titanate according to the present invention is preferablythose represented by the compositional formula Li_(x)Ti_(y)O₄, and morepreferably a single phase of lithium titanate. However, titanium oxidemay be slightly mixed in the range where the advantageous effects of theinvention are not impaired. As the values x and y in the compositionalformula, the value of x/y is preferably in the range of 0.5 to 2.Particularly preferable is a spinel type represented by thecompositional formula Li₄Ti₅O₁₂.

In the present invention, the surface of the secondary particle may becoated with at least one selected from inorganic compounds such assilica and alumina and organic compounds such as a surface active agentand a coupling agent. In these coating species, one thereof can becarried, two or more thereof can be laminated as a plurality of carryinglayers, or two or more thereof can be carried as a mixture or a complexproduct.

Alternatively, the inside and surface of the secondary particle of thelithium titanate can contain carbon. The containing of carbon ispreferable because electrical conductivity is improved. The amount ofcarbon to be contained is preferably in the range of 0.05 to 30% byweight in terms of C. At an amount less than the range, a desiredelectrical conductivity is not obtained. At an amount more than therange, inactive material components within an electrode are undesirablyincreased to reduce the capacity of the battery. More preferably, theamount of carbon to be contained is in the range of 0.1 to 15% byweight. The amount of carbon can be analyzed by the CHN analysis method,high frequency combustion method, or the like.

Alternatively, the secondary particle can contain a different metalelement other than titanium and lithium. The different metal element ispreferably magnesium, aluminum, zirconium, and the like. One or two ormore thereof can be used. The amount of the different metal element tobe contained is preferably in the range of 0.05 to 15% by weight interms of Mg, Al, or Zr. More preferably, the amounts of Al and Mg are inthe range of 0.05 to 10% by weight, and the amount of Zr is in the rangeof 0.1 to 10% by weight. The amounts of Al and Mg are still morepreferably in the range of 0.1 to 5% by weight. The amount of thedifferent metal element can be analyzed by the inductively coupledplasma (ICP) method, for example.

Next, the present invention is a process for production of a lithiumtitanate, comprising the steps of: drying and granulating a slurrycomprising a crystalline titanium oxide, a titanic acid compound, and alithium compound; and firing a resultant product to obtain lithiumtitanate secondary particles, wherein (1) the crystalline titanium oxidecomprising at least two kinds of crystalline titanium oxide particleshaving a different average particle sizes is used (hereinafter, referredto as a process (1) in some cases), and/or (2) an amount of thecrystalline titanium oxide to be used is more than 4 times larger thanthat of the titanic acid compound in the weight ratio in terms of TiO₂(hereinafter, referred to as a process (2) in some cases).

In the present invention, first, starting substances such as acrystalline titanium oxide, a titanic acid compound, and a lithiumcompound are added to a medium solution to prepare a slurry comprisingthese starting substances. For industrial advantages, the concentrationof the titanium component in the slurry is preferably in the range of120 to 300 g/L in terms of TiO₂, and more preferably in the range of 150to 250 g/L. As the medium solution, water or organic solvent such asalcohols, or a mixture thereof can be used. Industrially, water or anaqueous medium solution containing water as a main component ispreferably used. The order to add the starting substances to the mediumsolution is not limited. Preferably, the lithium compound is added tothe medium solution in advance, and subsequently the crystallinetitanium oxide and the titanic acid compound are added. Thereby, anincrease in the viscosity of the slurry and gelation of the slurry aresuppressed. The temperature of the medium solution containing thelithium compound is preferably in the range of 25 to 100° C. because thereaction of the titanic acid compound with the lithium compound at astage of preparing the slurry progresses and lithium titanate is easilyobtained during firing. The temperature is more preferably in the rangeof 50 to 100° C. The crystalline titanium oxide and the titanic acidcompound may be added to the medium solution containing the lithiumcompound separately, simultaneously, or as a mixture thereof.

In the case where the reaction is performed in water or an aqueousmedium solution containing water as a main component, a water-solublelithium compound such as lithium hydroxide, lithium carbonate, lithiumnitrate, and lithium sulfate is preferably used as the lithium compound.Among these, lithium hydroxide is preferable because of its highreactivity.

As the titanic acid compound, metatitanic acid represented by TiO(OH)₂or TiO₂.H₂O, orthotitanic acid represented by Ti(OH)₄ or TiO₂.2H₂O, or amixture thereof can be used, for example. The titanic acid compound isobtained by heat hydrolysis or neutralization hydrolysis of ahydrolyzable titanium compound. For example, metatitanic acid isobtained by heat hydrolysis of titanyl sulfate (TiOSO₄), neutralizationhydrolysis of titanium chloride under a high temperature, or the like.Orthotitanic acid is obtained by neutralization hydrolysis of titaniumsulfate (Ti(SO₄)₂) or titanium chloride (TiCl₄) under a low temperature.A mixture of metatitanic acid and orthotitanic acid is obtained byproperly controlling the temperature at neutralization hydrolysis oftitanium chloride. Examples of a neutralizer used in neutralizationhydrolysis include ammonia and ammonium compounds such as ammoniumcarbonate, ammonium sulfate, and ammonium nitrate, and the neutralizer,if used, can be decomposed and volatilized during firing. As thetitanium compound, other than the inorganic titanium compounds such astitanium sulfate, titanyl sulfate, and titanium chloride, organictitanium compounds such as titanium alkoxide may also be used.

As the crystalline titanium oxide, titanium dioxide represented by thecompositional formula TiO₂ is preferably used. The crystal systems oftitanium dioxide is not limited, and an anatase type, a rutile type, abrookite type, and the like can be used. The crystalline titanium oxidemay have a single crystal phase, or may be a mixed crystal phases thatcontains two or more crystal systems and may be partially amorphous. Theaverage particle size of the crystalline titanium oxide particles ispreferably in the range of 0.01 to 0.4 μm. At an average particle sizein the range, an increase in the viscosity of the slurry is suppressedeven in a high concentration. The crystalline titanium oxide can beobtained by a known process for production of a titanium dioxidepigment, for example, the so-called sulfuric acid method of heathydrolyzing and firing titanyl sulfate, the so-called chlorine method ofgaseous phase oxidizing titanium tetrachloride.

In the process (1), two or three or more of the crystalline titaniumoxide particles having different average particle sizes can be used. Ifother crystalline titanium oxide particles have an average particle size1.3 or more times, preferably 1.3 or more times and 40 or less times,more preferably 1.3 or more times and 10 or less times, and still morepreferably 1.3 or more times and 3.5 or less times larger than that ofthe crystalline titanium oxide particle having the smallest averageparticle size, the advantageous effects of the invention can be easilyobtained. The crystals systems of the respective particles may be thesame or different. The average particle size is the 50% median particlesize according to the electron microscopy, and a preferable averageparticle size of the crystalline titanium oxide particle having thesmallest average particle size is 0.01 to 0.20 μm. The average particlesizes of the other crystalline titanium oxide particles can be properlyadjusted according to the smallest average particle size by granulationinto a secondary particle. Alternatively, if the primary particle of thecrystalline titanium oxide is used, the average particle size ispreferably in the range of 0.05 to 0.40 μm. The weight of thecrystalline titanium oxide having an average particle size that is 1.3or more times larger than that of the crystalline titanium oxide havingthe smallest average particle size is in the range of 0.1 to 5 times theweight of the crystalline titanium oxide having the smallest averageparticle size. In the case where there exist a plurality of thecrystalline titanium oxides having the average particle size 1.3 or moretimes larger than that of the crystalline titanium oxide having theminimal average particle size, the total weight thereof is used as areference. The total amount of the crystalline titanium oxide particlesto be used is preferably in the range of 1 to 10 times larger than thatof the titanic acid compound in the weight ratio in terms of TiO₂because the lithium titanate can be produced with industrial advantages.

In the process (2), the amount of the crystalline titanium oxide to beused particularly has no upper limit as long as the amount is more than4 times and preferably 4.2 or more times larger than that of the titanicacid compound. The amount 10 or less times larger than that of thetitanic acid compound is preferable because the viscosity of the slurryis suitable for drying and granulation. The crystalline titanium oxidemay be one kind of crystalline titanium oxide particle, or two or morekind of crystalline titanium oxide particles having different averageparticle sizes or crystal systems.

The slurry is dried and granulated, and subsequently fired to obtain thelithium titanate. As a method for granulating with drying, a knownmethod can be used. Examples of the known method include (A) a method inwhich a slurry is spray-dried and granulated into a secondary particle,and (B) a method in which a solid contained in a slurry is dehydratedand dried, and then the solid thus dried is crushed and granulated intosecondary particles having a desired size. Particularly, the method (A)is preferable because control of the particle size is easy and aspherical secondary particle can be easily obtained. A spray drier usedfor spray drying can be properly selected from a disk type, a pressurenozzle type, a two fluid nozzle type, and a four fluid nozzle type, forexample, according to the properties and state of the slurry and theperformance of the spray drier. The particle size of the secondaryparticle is controlled as follows: for example, the concentration of thesolid content in the slurry is adjusted, or in the disk type spraydrier, the number of rotation of the disk is adjusted, or in thepressure nozzle type, two fluid nozzle type, and four fluid nozzle typespray driers, the spray pressure, the diameter of the nozzle, and theflow rate of each fluid are adjusted thereby to control the size ofdroplets of the solution to be sprayed. The properties and state of theslurry such as a concentration and viscosity are properly determinedaccording to the ability of the spray drier.

In the case where the slurry has a low viscosity and is difficult togranulate, an organic binder may be used in order to further facilitatecontrol of the particle size. Examples of the organic binder to be usedinclude (1) vinyl compounds (such as polyvinyl alcohol andpolyvinylpyrrolidone), (2) cellulose compounds (such as hydroxyethylcellulose, carboxymethyl cellulose, methyl cellulose, and ethylcellulose), (3) protein compounds (such as gelatin, gum arabic, casein,sodium caseinate, and ammonium caseinate), (4) acrylic acid compounds(such as sodium polyacrylate and ammonium polyacrylate), (5) naturalpolymer compounds (such as starch, dextrin, agar, and sodium alginate),(6) synthetic polymer compounds (such as polyethylene glycol), and atleast one selected from these can be used. Among these, more preferableare those containing no inorganic component such as soda because thoseare easily decomposed and volatilized by firing.

The firing temperature depends on the firing atmosphere or the like. Inorder to produce lithium titanate, the firing temperature may beapproximately 550° C. or more. In order to prevent sintering between thesecondary particles, the firing temperature is preferably 1000° C. orless. From the viewpoint of acceleration of production of Li₄Ti₅O₁₂ andimprovement of the rate characteristics, the firing temperature is morepreferably in the range of 550 to 850° C., and still more preferably inthe range of 650 to 850° C. The firing atmosphere can be properlyselected from in the air, a non-oxidizing atmosphere, and the like.After firing, if the obtained lithium titanate secondary particles aresintered and agglomerated, the obtained lithium titanate secondaryparticles may be crushed when necessary using a flake crusher, ahammermill, a pin mill, a bantam mill, a jet mill, or the like.

The present invention may further comprise the step of adding carbon tothe lithium titanate secondary particles. Examples of a specific methodof adding the carbon include (A) a method in which the slurry comprisinga crystalline titanium oxide, a titanic acid compound, and a lithiumcompound is granulated with drying, and then the resultant product isfired, followed by firing an obtained fired product again in thepresence of a carbon-containing substance, and (B) a method in which theslurry comprises a crystalline titanium oxide, a titanic acid compound,a lithium compound, and a carbon-containing substance, the slurry isdried and granulated, and the resultant product is fired. The firingtemperature in the presence of the carbon-containing substance ispreferably in the range of 150 to 1000° C. in the case of (A), and inthe range of 550 to 1000° C. in the case of (B) in which range thelithium titanate is easily produced. The firing atmosphere can beproperly selected from in the air, a non-oxidizing atmosphere, and thelike. Preferably, firing is performed under a non-oxidizing atmosphere.

Examples of the carbon-containing substance include carbon black,acetylene black, ketjen black, and organic compounds. The organiccompounds may be preheated and thermally decomposed for use. In the casewhere the organic compound is used, hydrocarbon compounds and/oroxygen-containing hydrocarbon compounds in which a component other thancarbon is difficult to remain are preferable. Examples of thehydrocarbon compounds include (1) alkane compounds (such as methane,ethane, and propane), (2) alkene compounds (such as ethylene andpropylene), (3) alkyne compounds (such as acetylene), (4) cycloalkanecompounds (such as cyclohexane), and (5) aromatic compounds (such asbenzene, toluene, and xylene). Examples of the oxygen-containinghydrocarbon compounds include (1) alcohol compounds (such as (a)monohydric alcohols (such as methanol, ethanol, and propanol), (b)dihydric alcohols (such as ethylene glycol), (c) trihydric alcohols(such as trimethylol ethane and trimethylol propane), (d) polyalcohols(such as polyvinyl alcohol)), (2) ether compounds (such as (a) ethermonomers (such as diethyl ether and ethyl methyl ether), (b) polyethers(such as polyethylene glycol, polyethylene oxide, and polypropyleneether)), (3) carboxylic acid compounds (such as (a) oxycarboxylic acids(such as citric acid and malic acid), (b) monocarboxylic acids (such asacetic acid and formic acid), (c) dicarboxylic acids (such as oxalicacid and malonic acid), (d) aromatic carboxylic acids (such as benzoicacid)), (4) aldehyde compounds (such as formaldehyde and acetaldehyde),(5) phenol compounds (such as phenol, catechol, and pyrogallol), and (6)saccharides (such as glucose, sucrose, and cellulose). In the case wheredrying and granulation are performed by spray drying, a compound servinga binder such as polyalcohols and polyethers can be selected as theorganic compound.

Moreover, a step of adding a different metal element other than titaniumand lithium to the lithium titanate secondary particles can be provided.Examples of a specific method of adding the different metal element tothe secondary particles include (A) a method in which a compound of adifferent metal element is added to the slurry comprising a crystallinetitanium oxide, a titanic acid compound, and a lithium compound, and (B)a method in which the slurry comprises a crystalline titanium oxidecontaining a different metal element, a titanic acid compound, and alithium compound, the slurry is dried and granulated, and the resultantproduct is fired. In the method (A), the compound of a different metalelement can be mixed with the crystalline titanium oxide or the titanicacid compound in advance. In the case of the crystalline titanium oxide,the surface of the particle may be coated with the compound of adifferent metal element to obtain a mixture. In the case of the titanicacid compound, a hydrolyzable titanium compound may be hydrolyzed in thepresence of the compound of a different metal element to obtain amixture. The crystalline titanium oxide containing a different metalelement for use in the method (B) is obtained by mixing the titaniumcompound with the compound of a different metal element and firing themixture. The compound of a different metal element is properly selectedfrom oxides, hydrous oxides, chlorides, carbonates, nitric acid salts,sulfuric acid salts, and the like of different metal elements, dependingon the methods (A) and (B).

Next, the present invention is an electrode active material comprisingthe lithium titanate. Moreover, the present invention is an electricitystorage device comprising an electrode containing the electrode activematerial mentioned above. Examples of the electricity storage devicespecifically include lithium batteries and lithium capacitors. Theseinclude an electrode, a counter electrode, a separator, and anelectrolyte solution. The electrode is obtained by adding a conductivematerial and a binder to the active material, and properly molding themixture or applying the mixture to a plate. Examples of the conductivematerial include carbon-containing substances such as carbon black,acetylene black, and ketjen black. Examples of the binder includefluorinated resins such as polytetrafluoroethylene, polyvinylidenefluoride, and fluorinated rubbers, rubber binders such as styrenebutadiene, and water-soluble resins such as carboxymethyl cellulose andpolyacrylic acid. In the case of the lithium batteries, the electrodeactive material mentioned above can be used as the positive electrode,and metallic lithium, a lithium alloy, or a carbon-containing substancesuch as graphite can be used as the counter electrode. Alternatively,the electrode active material mentioned above can be used as thenegative electrode, and lithium and transition metal complex oxides suchas lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide,and lithium vanadium oxide, and olivine compounds such as lithium ironphosphoric acid compound can be used as the positive electrode. In thecase of the capacitors, an asymmetric capacitor using the electrodeactive material and a carbon-containing substance such as graphite oractivated carbon can be formed. As the separator, a porous polyethylenefilm or the like is used in both cases. As the electrolyte solution, anordinary material can be used, for example, those obtained by dissolvinga lithium salt such as LiPF₆, LiClO₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiBF₄in a solvent such as propylene carbonate, ethylene carbonate, dimethylcarbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyllactone,and 1,2-dimethoxyethane.

Further, the present invention is another electricity storage devicecomprising an electrode which contains the electrode active material butwhich contains no conductive material. Lithium titanate has electricinsulation. For this reason, charge and discharge capacity isconventionally hard to obtain without using any conductive material,e.g., a carbon-containing substance such as carbon black, acetyleneblack, and ketjen black. In the electricity storage device according tothe present invention, however, sufficient charge and discharge capacityis practically obtained without using a conductive material. Moreover,the electricity storage device has excellent rate property. In thepresent invention, that the electrode “contains no conductive material”includes the state where a conductive material is not added to theelectrode, and the state where the inside and surface of the lithiumtitanate contain no conductive material such as carbon. The electrodeactive material used for the counter electrode of the electrode, thebinder, the electrolyte solution, and the like described above can beused.

EXAMPLES

Hereinafter, Examples according to the present invention will be shown,but the present invention will not be limited to these.

Example 1 Production Process (1))

To 340 mL of a 4.5-mol/L lithium hydroxide aqueous solution, 50 g of acrystalline titanium dioxide particle (a) (anatase form) having anaverage particle size of 0.10 μm and 50 g of a crystalline titaniumdioxide particle (b) (mixed crystal phase of an anatase type and arutile type) having an average particle size of 0.07 μm were added, anddispersed. While the slurry was stirred, the temperature of the solutionwas kept at 80° C., and 650 mL of an aqueous slurry prepared bydispersing 50 g of a titanic acid compound (orthotitanic acid) in termsof TiO₂ was added to obtain a slurry comprising a crystalline titaniumoxide, a titanic acid compound, and a lithium compound. The slurry wasspray-dried using a GB210-B spray drier (made by Yamato Scientific Co.,Ltd.) under the condition of an inlet temperature of 190° C. and anoutlet temperature of 80° C. to obtain a dried and granulated product.Then, the dried and granulated product was fired in the air at atemperature of 700° C. for 3 hours to obtain a lithium titanate (SampleA) according to the present invention represented by the compositionalformula Li₄Ti₅O₁₂. The average particle size of the crystalline titaniumdioxide particle was measured using a transmission electron microscopeH-7000 and an image diffractometer LUZEX IIIU (both are made by Hitachi,Ltd.).

Example 2 Production Process (1))

To 340 mL of a 4.5-mol/L lithium hydroxide aqueous solution, 85.7 g of acrystalline titanium dioxide particle (b) (mixed crystal of an anataseform and a rutile form) having an average particle size of 0.07 μm and21.5 g of a crystalline titanium dioxide particle (c) (mixed crystal ofan anatase form and a rutile form) having an average particle size of0.13 μm were added, and dispersed. While the slurry was stirred, thetemperature of the solution was kept at 80° C., 420 mL of an aqueousslurry prepared by dispersing 42.9 g of a titanic acid compound(orthotitanic acid) in terms of TiO₂ was added to obtain a slurrycomprising a crystalline titanium oxide, a titanic acid compound, and alithium compound. Subsequently, the dried and granulated product wasprepared and fired in the same manner as in Example 1 to obtain alithium titanate (Sample B) according to the present inventionrepresented by the compositional formula Li₄Ti₅O₁₂.

Example 3 Production Process (1))

50 g of the lithium titanate obtained in Example 1 (Sample A) wasuniformly mixed with 2.5 g of polyethylene glycol, and the mixture wasfired under a nitrogen atmosphere at a temperature of 500° C. for 2hours to obtain a lithium titanate (Sample C) according to the presentinvention. According to analysis using a CHN elemental analyzer Vario ELIII (made by Elementar Analysensysteme GmbH), it turned out that SampleC contains 0.80% by weight of carbon in terms of C.

Example 4 Production Process (1))

A lithium titanate (Sample D) according to the present inventioncontaining 2.1% by weight of magnesium in terms of Mg was obtained inthe same manner as in Example 1 except that the amounts of thecrystalline titanium dioxide particles (a) and (b) and the titanic acidcompound to be used in Example 1 each were 53.2 g in terms of TiO₂, theamount of the aqueous slurry of the titanic acid compound to be addedwas 680 mL, and 8.8 g of magnesium hydroxide (containing 3.5 g of Mg)was further added. The amount of magnesium was measured using an ICPoptical emission spectrometer SPS-3100 (made by Seiko Instruments Inc.).

Example 5 (Production Process (1))

A lithium titanate (Sample E) according to the present inventioncontaining aluminum was obtained in the same manner as in Example 1except that the amounts of the crystalline titanium dioxide particles(a) and (b) and the titanic acid compound to be used in Example 1 eachwere 54.5 g in terms of TiO₂, the amount of the aqueous slurry of thetitanic acid compound to be added was 690 mL, and 12.3 g of aluminumhydroxide (containing 4.1 g of Al) was further added. The content ofaluminum in Sample E was measured in the same manner as in Example 4,and it was 2.3% by weight in terms of Al.

Example 6 Production Process (1))

A lithium titanate (Sample F) according to the present inventioncontaining 8.4% by weight of zirconium in terms of Zr was obtained inthe same manner as in Example 1 except that the amounts of thecrystalline titanium dioxide particles (a) and (b) and the titanic acidcompound to be used in Example 1 each were 53.2 g in terms of TiO₂, theamount of the aqueous slurry of the titanic acid compound to be addedwas 680 mL, and 9.3 g of zirconium oxide (containing 6.9 g of Zr) wasfurther added.

Example 7 Production Process (2))

To 340 mL of a 4.5-mol/L lithium hydroxide aqueous solution, 125 g ofthe crystalline titanium dioxide particle (b) having an average particlesize of 0.07 μm was added, and dispersed. While the slurry was stirred,the temperature of the solution was kept at 80° C., 250 mL of an aqueousslurry prepared by dispersing 25 g of the titanic acid compound(orthotitanic acid) in terms of TiO₂ was added to obtain a slurrycomprising a crystalline titanium oxide, a titanic acid compound, and alithium compound. Subsequently, the dried and granulated product wasprepared and fired in the same manner as in Example 1 to obtain alithium titanate (Sample G) according to the present inventionrepresented by the compositional formula Li₄Ti₅O₁₂.

Comparative Example 1

To 340 mL of a 4.5-mol/L lithium hydroxide aqueous solution, 75 g of thecrystalline titanium dioxide particle (b) having an average particlesize of 0.07 μm was added, and dispersed. While the slurry was stirred,the temperature of the solution was kept at 80° C., and 720 mL of anaqueous slurry prepared by dispersing 75 g of the titanic acid compound(orthotitanic acid) in terms of TiO₂ was added to obtain a slurrycomprising a crystalline titanium oxide, a titanic acid compound, and alithium compound. Subsequently, the dried and granulated product wasprepared and fired in the same manner as in Example 1 to obtain alithium titanate (Sample H) for comparison represented by thecompositional formula Li₄Ti₅O₁₂.

Comparative Example 2

A lithium titanate (Sample I) for comparison represented by thecompositional formula Li₄Ti₅O₁₂ was obtained in the same manner as inComparative Example 1 except that the amount of the crystalline titaniumdioxide particle (b) to be used in Comparative Example 1 was 111.5 g,the amount of the titanic acid compound (orthotitanic acid) to be usedin Comparative Example 1 was 38.5 g in terms of TiO₂, and 375 mL of theaqueous slurry was added.

Comparative Example 3

To 340 mL of a 4.5-mol/L lithium hydroxide aqueous solution, 1500 mL ofan aqueous slurry prepared by dispersing 150 g of the titanic acidcompound (orthotitanic acid) in terms of TiO₂ was added, and thetemperature of the solution was kept at 80° C. while the solution wasstirred. Thus, a slurry comprising a titanic acid compound and a lithiumcompound was obtained. Subsequently, the dried and granulated productwas prepared and fired in the same manner as in Example 1 to obtain alithium titanate (Sample J) for comparison represented by thecompositional formula Li₄Ti₅O₁₂.

Examples 8 to 14

Each of the lithium titanates (Samples A to G) obtained in Examples 1 to7, acetylene black powder as a conductive material, and a polyvinylidenefluoride resin as a binder were mixed in a weight ratio of 100:5:7, andkneaded in a mortar to prepare a paste. The paste was applied onto analuminum foil, and dried at a temperature of 120° C. for 10 minutes.Then, the aluminum foil was blanked out into a circular shape having adiameter of 12 mm, and pressed at 17 MPa to form a working electrode.The amount of the active material contained in the electrode was 3 mg.

The working electrode was vacuum dried at a temperature of 120° C. for 4hours, and incorporated as a positive electrode into a sealable coincell in a glovebox at a dew point of −70° C. or less. A stainless steel(SUS316) coin cell having an outer diameter of 20 mm and a height of 3.2mm was used. The negative electrode prepared by molding metallic lithiumhaving a thickness of 0.5 mm into a circular shape having a diameter of12 mm was used. As a nonaqueous electrolytic solution, a mixed solutionof ethylene carbonate and dimethyl carbonate (mixed in a volume ratio of1:2) in which LiPF₆ was dissolved such that the concentration might be 1mol/L was used.

The working electrode was disposed in the lower can of the coin cell. Aporous polypropylene film was disposed on the working electrode as aseparator. A nonaqueous electrolytic solution was dropped over theporous polypropylene film. On the porous polypropylene film, thenegative electrode, and a spacer with a thickness of 0.5 mm foradjusting the thickness and a spring (both made of SUS316) weredisposed, and covered with the upper can with a propylene gasket. Theedge of the outer periphery was caulked to be sealed. Thus, electricitystorage devices (Samples K to Q) according to the present invention wereobtained. The respective samples are Examples 8 to 14.

Example 15

An electricity storage device (Sample R) according to the presentinvention was obtained in the same manner as in Example 8 except thatwithout using acetylene black in Example 8, Sample A and thepolyvinylidene fluoride resin were mixed in a weight ratio of 100:7 toprepare a paste.

Comparative Examples 4 to 6

Electricity storage devices (Samples S to U) for comparison wereobtained in the same manner as in Example 8 except that instead ofSample A in Example 8, Samples H to J obtained in Comparative Examples 1to 3 were used. The respective samples are Comparative Examples 4 to 6.

Example 16

The lithium titanate (Sample A) obtained in Example 1, acetylene blackpowder as the conductive agent, and a polyvinylidene fluoride resin asthe binder were mixed in a weight ratio of 100:3:10, and kneaded in amortar to prepare a paste. The paste was applied onto an aluminum foil,and dried at a temperature of 120° C. for 10 minutes. Then, the aluminumfoil was blanked out into a circular shape having a diameter of 12 mm,and pressed at 17 MPa to form a working electrode. The amount of theactive material contained in the electrode was 4 mg.

Commercially available lithium manganate (M01Y01: made by Mitsui Mining& Smelting Co., Ltd.) as the active material, acetylene black as aconductive material, and a polyvinylidene fluoride resin as a binderwere kneaded in a weight ratio of 100:10:10. The kneaded product wasapplied onto an aluminum foil current collector, and dried at atemperature of 120° C. for 10 minutes. The current collector was cut outinto a circular shape having a diameter of 12 mm, and pressed at 17 MPato obtain a positive electrode. The amount of the active materialcontained in the electrode was 8 mg.

Each of these electrodes was vacuum dried at a temperature of 120° C.for 5 hours, and incorporated into a sealable coin cell for a test in aglovebox at a dew point of −70° C. or less. A stainless steel (SUS316)cell for evaluation having an outer diameter of 20 mm and a height of3.2 mm was used. The lithium manganate electrode was disposed as thepositive electrode in the lower can of the cell for evaluation. A porouspolypropylene film was disposed on the positive electrode as theseparator. On the porous polypropylene film, the working electrode asthe negative electrode, and a spacer with a thickness of 1.0 mm foradjusting the thickness and a spring (both made of SUS316) weredisposed. Over them, a mixed solution of ethylene carbonate and dimethylcarbonate (mixed in a volume ratio of 1:2) in which LiPF₆ was dissolvedto have a concentration of 1 mol/L was dropped as a nonaqueouselectrolytic solution. The lower can was covered with the upper can witha propylene gasket, and the edge of the outer periphery was caulked tobe sealed. Thus, an electricity storage device according to the presentinvention (Sample V) was obtained.

Comparative Example 7

An electricity storage device for comparison (Sample W) was obtained inthe same manner as in Example 16 except that instead of Sample A inExample 16, Sample J obtained in Comparative Example 3 was used. This isComparative Example 7.

Evaluation 1: Measurement of Amounts of Nitrogen Adsorbed and Desorbed

In the lithium titanates (Samples A to G and J) obtained in Examples 1to 7 and Comparative Example 3, the amounts of nitrogen adsorbed anddesorbed were measured using a high precision automatic gas adsorptionamount measuring apparatus (BELSORP-mini II: made by BEL Japan, Inc.).Approximately 1 g of a sample was placed in a measurement cell vacuumdegassed approximately for 1 day. Using a pre-treatment apparatus(BELLPREP-vac II: made by BEL Japan, Inc.), vacuum degassing wasperformed at a temperature of 150° C. for 3 hours. Subsequently, underthe liquid nitrogen temperature (77 K), nitrogen gas with high puritywas adsorbed and desorbed to obtain adsorption and desorption isotherms.The adsorption and desorption isotherms of Sample A are shown in FIG. 1.In FIG. 1, “ADS” represents the adsorption isotherm, “DES” representsthe desorption isotherm, “p/p0” represents a relative pressure, “V_(a)”represents the amount adsorbed, and “V_(d)” represents the amountdesorbed. The amounts (V_(a(0.99)), V_(a(0.50))) of nitrogen adsorbed ata relative pressure of 0.99 and at that of 0.50, and the difference(ΔV_(d-a(p))) between the amount of nitrogen desorbed and the amount ofnitrogen adsorbed when the measurement was made at an interval of therelative pressure of 0.05 in the range of 0.45 to 0.90 are shown inTable 1. It is demonstrated that the lithium titanates according to thepresent invention have V_(a(0.99)) of 50 cm³ (STP)/g or more, and havemacropores on the surface of the secondary particle. It is alsodemonstrated that lithium titanates according to the present inventionhave few mesopores and micropores because V_(a(0.50)) is 10 cm³ (STP)/gor less, ΔV_(d-a(p)) does not continuously take values of 5 cm³ (STP)/gor more, and the adsorption and desorption isotherms have no hysteresis.

TABLE 1 Sample V_(a(0.99)) V_(a(0.50)) ΔV_(d−a(0.45)) ΔV_(d−a(0.50))ΔV_(d−a(0.55)) ΔV_(d−a(0.60)) Example 1 A 80.7 4.9 0.1 0.1 0.1 0.1Example 2 B 76.1 3.8 0.1 0.1 0.1 0.1 Example 3 C 89.7 4.0 0.6 0.7 0.80.9 Example 4 D 168.4 7.2 0.8 1.0 1.2 1.3 Example 5 E 77.6 3.5 1.0 1.11.5 1.4 Example 6 F 58.4 3.8 0.2 0.2 0.3 0.3 Example 7 G 85.1 3.7 0.10.1 0.1 0.1 Comparative J 19.6 0.0 0.0 0.1 0.1 0.1 Example 3ΔV_(d−a(0.65)) ΔV_(d−a(0.70)) ΔV_(d−a(0.75)) ΔV_(d−a(0.80))ΔV_(d−a(0.85)) ΔV_(d−a(0.90)) Example 1 0.1 0.1 0.2 0.1 0.3 0.7 Example2 0.1 0.2 0.3 0.2 0.1 0.8 Example 3 1.0 1.0 1.1 1.0 1.3 1.0 Example 41.4 1.5 1.7 1.8 2.7 1.6 Example 5 1.5 1.7 1.8 1.9 1.9 2.0 Example 6 0.30.3 0.4 0.5 0.3 0.9 Example 7 0.1 0.2 0.2 0.2 0.2 0.4 Comparative 0.10.1 0.2 0.2 0.2 0.0 Example 3 (Units: cm³ (STP)/g)Evaluation 2: Evaluation of Rate Property in Electricity Storage DeviceUsing Lithium Titanate as Positive Electrode Active Material

In the electricity storage devices (Samples K to U) obtained in Examples8 to 15 and Comparative Examples 4 to 6, a discharge capacity wasmeasured at a variety of the current amount, and a capacity retention(%) was calculated. The measurement was made at a voltage in the rangeof 1 to 3 V, at a charge current of 0.25 C, and at a discharge currentin the range of 0.25 C to 30 C. The environmental temperature was 25° C.The capacity retention was calculated by the expression(X_(n)/X_(0.25))×100 wherein the measured value of the dischargecapacity at 0.25 C was X_(0.25), and the measured value thereof in therange of 0.5 C to 30 C was X_(n). Here, 1 C refers to the current valuethat can be fully charged in 1 hour, and in the present evaluation, 0.48mA is equivalent to 1 C. A higher capacity retention means higher ratecharacteristics. The result is shown in Table 2. It turned out that theelectricity storage devices according to the present invention have acapacity retention of 70% or more at 30 C, and high ratecharacteristics. The electricity storage device according to the presentinvention containing no conductive material has high ratecharacteristics equal to those of the electricity storage devicecontaining a conductive material.

TABLE 2 Capacity retention (%) Sample 0.5 C 1 C 5 C 10 C 20 C 30 CExample 8 K 100.0 99.6 97.6 93.1 86.0 82.6 Example 9 L 100.0 98.0 92.485.8 76.5 71.6 Example 10 M 100.0 99.0 92.0 87.8 79.5 71.3 Example 11 N100.0 98.9 93.8 90.5 80.9 73.9 Example 12 O 99.7 99.0 96.7 91.8 84.876.3 Example 13 P 100.0 99.9 98.6 94.6 86.5 82.5 Example 14 Q 100.0 99.296.0 94.6 82.3 88.7 Example 15 R 100.0 99.3 93.6 91.8 89.6 87.1Comparative S 100.0 91.5 75.1 67.7 55.4 47.7 Example 4 Comparative T100.0 97.5 89.5 83.9 72.1 64.9 Example 5 Comparative U 100.0 74.3 41.328.8 16.8 10.7 Example 6

Evaluation 3 Evaluation of Rate Property in Electricity Storage DeviceUsing Lithium Titanate as Negative Electrode Active Material

In the electricity storage devices (Samples V and W) obtained in Example16 and Comparative Example 7, a discharge capacity was measured at avariety of the current amount, and a capacity retention (%) wascalculated. The electricity storage device was produced, and aged for 3hours, and conditioning was performed by 2-cycle charging anddischarging at 0.25 C. Subsequently, the measurement was made at avoltage in the range of 1.5 to 2.8 V, at a discharge current of 0.25 C,and at a charge current in the range of 0.25 C to 10 C. Theenvironmental temperature was 25° C. The capacity retention wascalculated by the expression (X_(ii)/X_(0.25))×100 wherein the measuredvalue of the discharge capacity in charging at 0.25 C was X_(0.25), andthe measured value thereof in the range of 0.5 C to 10 C was X_(n).Here, 1 C refers to the current value that can be fully charged in 1hour, and in the present evaluation, 0.64 mA is equivalent to 1 C. Theresult is shown in Table 3. It turned out that the electricity storagedevice according to the present invention has a capacity retention at 10C of 70% or more, and has excellent rate property even if the lithiumtitanate according to the present invention is used as the negativeelectrode active material.

TABLE 3 Capacity retention (%) Sample 0.5 C 1 C 2 C 5 C 10 C Example 16V 99.0 98.6 96.5 92.2 70.9 Comparative W 96.9 91.3 84.0 68.3 41.9Example 7

INDUSTRIAL APPLICABILITY

The lithium titanate according to the present invention has goodproperties for battery particularly excellent rate property and isuseful for an electricity storage device.

The invention claimed is:
 1. A process for producing a lithium titanate,comprising the steps of: drying and granulating a slurry comprising acrystalline titanium oxide, a titanic acid compound, and a lithiumcompound; and firing a resultant product to obtain a lithium titanatesecondary particle, wherein (1) the crystalline titanium oxide comprisesat least two crystalline titanium oxide particles having differentaverage particle sizes wherein the crystalline titanium oxide particleshaving the smallest average particle size and other crystalline titaniumoxide particles have an average particle size 1.3 or more times largerthan that of the crystalline titanium oxide particles having thesmallest average particle size are used.
 2. The process for producingthe lithium titanate according to claim 1, further comprising a step tomake a carbon containing lithium titanate secondary particle.
 3. Theprocess for producing the lithium titanate according to claim 2,wherein:(A) the slurry comprising the crystalline titanium oxide, thetitanic acid compound, and the lithium compound is granulated withdrying, and a resultant product is fired, followed by firing an obtainedfired product again in the presence of a carbon-containing substance; or(B) the slurry comprises the crystalline titanium oxide, the titanicacid compound, the lithium compound, and a carbon-containing substanceis granulated with drying, and a resultant product is fired.
 4. Theprocess for producing the lithium titanate according to claim 1, furthercomprising a step to make the lithium titanate secondary particlecontain an additional metal element other than lithium and titanium. 5.The process for producing the lithium titanate according to claim 4,wherein: (A) the compound of the different metal element is added to theslurry comprising the crystalline titanium oxide, the titanic acidcompound, and the lithium compound; or (B) the slurry comprises thecrystalline titanium oxide containing the different metal element, thetitanic acid compound, and the lithium compound, and the slurry is driedand granulated, and then the resultant product is fired.
 6. The processfor producing the lithium titanate according to claim 1, furthercomprising a step to make the lithium titanate secondary particlecontain an additional metal element selected from the group consistingof magnesium, aluminum and zirconium.
 7. The process for producing thelithium titanate according to claim 1, wherein an amount of thecrystalline titanium oxide to be used is more than 4 times larger thanthat of the titanic acid compound in a weight ratio in terms of TiO2.